[playful piano music]
[narrator] As a little kid,
a single day can last forever.
As a teenager, you can live what feels like a whole lifetime
in a single summer.
Your twenties stretch on and on and on.
And then suddenly, you can't believe you're somehow 40.
And the pace only picks up from there.
Our time warps on small scales too.
So, you might be aware of phrases like
"Time flies when you're having fun." "Time drags when you're bored."
[narrator] Studies show marijuana can slow down time,
and alcohol and cocaine can speed it up.
Music can do that too.
That hold music might feel like it's making every second torture.
-[woman] Thank you for holding. -[narrator] But silence…
[no audio]
…is worse.
Now, what I was particularly interested in was "Why?"
[narrator] That's a great question.
'Cause it seems like time passes at a steady, relentless tick,
but it also doesn't feel that way.
It feels like the day after day gallops and goes by so quickly.
It's weekend to weekend, and there's all this work happening all the time.
My days do go by,
seem to go by, quicker. And I'm not sure why.
[narrator] So why do we experience time the way we do?
And how can we take back some control?
Tick, tock, tick, tock, tick, tock, tick, tock.
[man 1] To study time, we must venture beyond the ordinary.
And when we do, it opens up a whole new realm.
Time and space.
[man 2] Just suppose we could slow down time.
Or speed it up.
[man 3] To the question, "What is time?"
there is no one answer.
[Gordon-Levitt] In 1962,
a young geologist went on an expedition into the French Alps.
He went with a team, but Michel Siffre, just 23 years old,
would face the real journey alone.
Michel wanted to see what it would be like
to live away from all people,
all clocks, and even the rising and setting of the sun,
by living in a cave for two months.
This was one of the first studies
of the human sleep-wake cycle
in the absence of any external signals about the time of day.
[Gordon-Levitt] As he described it in an interview later,
it was an existence beyond time.
I was beyond time, without any time cues.
No television, no radio. Nothing.
No watch.
Uh…
You live following your mind.
[Gordon-Levitt] Every time Michel woke up, ate, and went to sleep,
he'd call up his colleagues stationed at the cave's entrance.
Sometimes, he was awake for just six hours
and went back to sleep.
Or he could stay up for 27 hours in a row.
But on average, he fell into a sleep-wake cycle
of roughly 24 hours.
It was a key insight.
Away from any external cues about the time of day,
his body kept track of it.
Our most ancient clocks
are inside of us.
They're called circadian clocks.
"Circa" meaning approximately, and "dian" meaning day,
because they tell time on the scale of 24 hours.
Interestingly, most cells in our body have a circadian clock.
[Gordon-Levitt] They make special proteins specifically to tell time.
And when they reach a certain limit…
[Buonomano] That protein can turn off the synthesis of itself.
[Gordon-Levitt] And then those proteins are destroyed,
and the cell fires up again.
And that cycle happens every 24 hours.
Our bodies contain billions of these tiny loops.
[Buonomano] But there is a master clock in the human brain,
and that's located in the hypothalamus,
in an area called the suprachiasmatic nucleus.
That's in charge in many ways
of synchronizing all the other clocks in our bodies.
[Gordon-Levitt] Every mammal ever studied
has a 24-hour internal clock.
Every bird,
and reptile,
and fish,
and insect.
In fact, you don't even need a brain.
Plants have a circadian clock,
and even single-cell bacterium have a circadian clock.
[Gordon-Levitt] According to one theory, billions of years ago,
when the first glimmer of life emerged,
cells that divided during the day were damaged by the sun's UV rays.
But the cells that learned to divide at night
survived.
In other words, keeping time
may have been fundamental
to the evolution of life on Earth.
But circadian clocks are pretty basic.
They just keep our bodily functions on a rhythm,
telling us when to eat, sleep, wake up.
A few other things.
And it resets every day.
Your ability to keep track of time passing,
day to day, month to month,
year to year,
is something else entirely.
On September 14th, Michel Siffre's team called him up
to announce that his two months in the cave were over.
Siffre thought it was a joke.
By his count, it was only August 20th.
His body had kept track of the days pretty well,
but his mind had lost an entire month.
Fast forward 60 years.
Happy New Year, 2020.
[Gordon-Levitt] And the whole world experienced
a kind of isolation experiment.
When the UK first went into lockdown, I had a six-month-old baby,
and I also have a six-year-old and an eight-year-old child.
And my experience of time was
that it was ridiculous that there were 24 hours in the day.
Time was dragging so slowly for me.
I wondered, "Is this a much more widespread phenomena?"
[Gordon-Levitt] So she did a survey of nearly 700 people across the UK.
[Ogden] The majority of people experienced a significant distortion to time.
[Gordon-Levitt] 81% of them.
And interestingly, it was almost evenly split.
Around half, like Ruth, felt like time was dragging,
and the other half felt like time had sped up.
[Ogden] So generally, being bored, being sad,
and being depressed are all associated
with the slowing of the passage of time during coronavirus.
[Gordon-Levitt] Studies like this in other countries
have found similar results.
In France, Italy, Argentina, and there are currently studies going on in Iraq.
So globally, it would seem that we have
profound distortion to the passage of time during coronavirus.
During a pandemic, we know it's gonna be over some day, but it's so far away
that you have to consider that this is now where you live,
this is your life, this is your reality.
[Gordon-Levitt] Scott Kelly was more prepared than most for that experience.
Back in 2015, he launched into space
and stayed there a whole year,
the longest manned mission in NASA history,
and he returned to Earth a national hero.
Because a year living in a high-tech aluminum can
250 miles above Earth
is a long time.
I consciously tried not to count the days I was in space,
because I knew over the course of a year that would get kind of irritating.
[Gordon-Levitt] But then one day near the end,
the Russian cosmonaut Mikhail Kornienko said to him…
"Hey, Scott, do you know what day it is?"
The day when we only have ten days left in space.
And then every day after that, he asked how many days we had left.
So he counted down from ten to zero.
It drove me a little bit crazy 'cause it wasn't part of my plan.
[Gordon-Levitt] And you don't need to be an astronaut,
or live through a pandemic,
to know that stress can make things drag out.
We know that emotion is one of the things that really distorts our sense of time.
[Gordon-Levitt] It doesn't really affect our circadian clocks,
but it seems like there's some other kind of clock in our brains,
something like a stopwatch that consciously tracks the time for us.
But the stopwatch in this case isn't a machine.
It's more like a human.
When it's bored or lonely,
it obsessively counts every beat.
And when it's really stressed, it might even count some twice.
And then on the flip side, when we're busy, relaxed, and social,
our minds are doing other stuff, and we skip a lot of beats,
and time seems to whiz by.
I really like this idea, because it makes sense
that we have some sort of clock-like system in the brain.
But the problem with this idea is
that we've really struggled to locate where this clock is.
[Gordon-Levitt] We've tried to find it,
putting people in MRIs and asking them to count time passing.
And there isn't one area that consistently lights up.
Our sense of time seems to be embedded across our brain.
So we talk about our sense of time,
but, of course, we don't have an organ that detects time.
[Gordon-Levitt] We can't see or hear
or smell or touch it.
We have no direct way to sense it at all.
So in many ways, our sense of time
is a creation, a construct, of the human mind.
[Gordon-Levitt] For centuries, we assumed
that time had to be out there,
ticking away just like it existed in our heads.
It's hard to even imagine another option.
But now, many scientists believe
we only experience a tiny sliver
of what time really is.
And that the reality of time
is something much bigger.
Something more like
space.
We already sort of think of time like it's space,
although it depends on your culture,
or rather, your writing tradition.
In you live in the West, you imagine time moving left to right,
like words on a page.
In Israel, the past is on the right and the future on the left,
'cause that's the way you write Hebrew.
And in Mandarin, you write vertically,
which is why the past is up and the future is down.
But everywhere we think of time as having a direction,
as coordinates on a map.
It's like putting a grid, right? You put a grid on space.
This is so many grid marks this way, so many grid marks that way,
and so many grid marks going up. Then you can attach a time to it.
You are at this grid position at this time,
and that grid position at that time.
[Gordon-Levitt] So if space has three dimensions,
you can think of time as the fourth.
Four dimensions is hard to imagine. Also, time is weird.
But anyway, you put these four dimensions together, and you call it space-time.
[Gordon-Levitt] Where space and time
are woven together
in a single, stretchy fabric.
And that fabric is warped all over the place.
[Balasubramanian] Matter and energy can bend space.
And, even more dramatically, can bend time.
[Gordon-Levitt] So on Earth, time ticks a bit more slowly
than it does out in space.
And time on the Sun
ticks even more slowly than that.
And time also slows down
at really fast speeds.
This all happens at a scale too big for us tiny humans
to ever feel.
But by the 1970s,
we had invented atomic clocks
that were precise enough to test it.
We put them on a plane and flew them around the world.
And sure enough,
those clocks said that less time
had passed.
And if time moves more slowly at faster speeds,
that means a person would age more slowly too.
For example, if you're an astronaut
who's spent a lot of time zooming around Earth,
while your twin brother, who's also an astronaut,
stayed on the ground.
I am now, like, three milliseconds younger
than him.
I used to be, like, six minutes younger, approximately,
and now I am six minutes and three milliseconds younger,
so I got that going for me, I guess.
[Gordon-Levitt] But if time is a dimension, just like space,
that raises a pretty huge question.
Clearly, you can move to the right or the left in any spatial direction.
So why can't I move up or down in time,
or to the later times and the earlier times?
This is a really complicated and interesting question.
Because to the best of our knowledge,
it's not possible to move backward in time.
[Gordon-Levitt] But if all the space around us,
forwards and backwards,
is just as real as where we're sitting,
then the past and the future
should be just as real as the present.
As if it's all already recorded.
The stuff that already happened.
Time flies when you're having fun.
The stuff that hasn't happened yet.
I think the best holiday is New Year's Eve.
[Gordon-Levitt] You can't say it's all happening now,
because only now is now.
We don't even exactly know the vocabulary to use.
[Gordon-Levitt] But you could say,
just like everything that's happening now is happening somewhere,
everything in the past and future could be happening
"somewhen."
An eccentric young man,
Albert Einstein,
first came up with these theories back in 1905.
And 50 years later,
when his lifelong collaborator and close friend
Michele Besso passed away,
Einstein wrote a letter to his family.
"Now he has departed from this strange world
a little ahead of me."
"That means nothing."
"People like us, who believe in physics,
know that the distinction between past, present, and future
is only a stubbornly persistent illusion."
Now, this idea of time
is hard to swallow for a few reasons.
First, there's the small issue of free will.
If the future is already written,
that means whatever we do,
it's not up to us.
The recording knew it was going to happen anyway.
So that's pretty spirit-crushing.
And while we can put clocks on planes
and prove that time warps,
we can't send those planes into the past or into the future,
so we can never really prove that the past or the future is real.
In fact, we can hardly imagine what that even means.
There are a lot of videos and documentaries that have tried
to help us make sense of it,
using all kinds of metaphors,
like a DVD record,
or a vinyl record, or…
A loaf of bread.
[Gordon-Levitt] But it's actually pretty useless to try to imagine this.
We evolved from those single-cell organisms
that learned to tell day from night.
And somewhere along the line, we developed our particular sense of time,
because that helped us survive too,
and it's the only window into time we'll ever have.
And it isn't just one window,
because we also have an awareness of the past.
Which is a whole other sense of time.
In many ways, the brain's main function
is to use the past
to predict the future.
So the evolutionary advantage of that
is, of course, very, very profound.
[Gordon-Levitt] Revisiting the past to imagine the future.
It's how we first realized that stars in the sky moved in predictable ways.
And developed clocks and calendars to track them.
It's what allowed us to organize,
coordinate with each other,
and build civilizations.
The ability to engage in mental time travel is what makes Homo sapiens sapien.
[Gordon-Levitt] And the fact that time is a sense that's easily warped,
in some ways, it's a gift,
because we have the power to speed it up, or slow it down,
depending on how we choose to spend it.
We search our memory,
and the more memories that we've accumulated during a period of time,
the longer we believe that period of time to have lasted for.
[Gordon-Levitt] Michel Siffre
felt like he only spent one month in a cave
because he had fewer memories worth storing.
He said in an interview later,
"When you're surrounded by night, your memory doesn't capture time."
"You forget."
And as you get older and busier, and settle into a routine,
you can also forget.
Just the emails will sort of keep you busy forever.
So that boom, boom, boom
makes it feel like the time has gotten very compressed.
[Gordon-Levitt] But for many people,
childhood felt like it lasted the longest.
If you think when you're a child, you're learning lots of new skills.
You're learning how to socialize and how to play,
and you're taking on lots of educational knowledge from school.
And I think the perception of whether it's moving fast or slow
has something to do with what you notice.
Children notice everything. They're very much in the present.
And having that process where every moment counts
I think gives you the sense
of a longer period of time that slows everything down.
[Gordon-Levitt] In fact, studies show just being in nature
makes time pass more slowly.
Being social, on the other hand, can make time fly by.
One of the things I really wanted to capture in my study
was the thing that had really changed in the UK as a result of COVID.
We couldn't go out and socialize with friends and family.
We couldn't meet people in the park.
So perhaps the most interesting variable that I also studied
was how satisfied they were with social interaction,
because it didn't matter how many people you lived with.
[Gordon-Levitt] What mattered was how socially satisfied you felt.
And all the celebrations people missed in the pandemic.
The ones we use to mark the years
may make time whiz by in the moment,
but they also force us to notice time passing
and create landmarks in our memories.
Making them one of the best ways
to live a long life in retrospect.
You know, birthdays are great in space.
What's really, I think, the best holiday is New Year's Eve,
because New Year's is something that everyone celebrates from every culture.
[Gordon-Levitt] On the very same day every year,
the entire world comes together
to mark another spin of our planet around the sun.
Counting the beats of time's steady march forward
as the future rushes into the present
and after a fleeting moment of now…
[crowd] Three, two, one…
[cheering]
[Gordon-Levitt] …vanishes into the past.
That young geologist Michel Siffre
was a 60-year-old man in 1999
when he took off his watch with a familiar gesture,
waved goodbye to a crowd of reporters,
and disappeared into a cave for another isolation experiment.
Siffre greeted the new millennium
some 3,000 feet below ground,
with champagne and foie gras.
And there was no one there to tell him that, in fact, in the world above ground,
it was already January 4th.
[closing theme music playing]